I'm not really sure why this thread has taken the turn it has, so we'll try to get it back on track.
1) The old rule of thumb is that bermudagrass fertilized at 1#N/M/growing month will produce a good surface. You could go higher or lower, but most universities broadly recommend that bermudagrass lawns be fertilized at 3 to 5#N/M/yr.
2) What analysis do you use? The answer will vary based on your equipment and other nutrient needs. What are your soil test P and K values? If you have a small enough customer base, maybe you could test all the lawns. If your customer base is too big for that, maybe you can test a random sample of lawns to get an idea of what you're facing.
If you need no P and not much K, that 36-0-8 may work well for you. Understand that an 18-0-4 analysis will do the same thing as the 36-0-4, but you'll need more of it.
3) How many times/yr do you apply? This part is kind of customizable. If your business needs dictate frequent applications, you can apply monthly and provide top notch service for your customers. If your business dictates 8 week applications, you can do this, too, and still get great results.
4) Turf color may have something to do with nutrient status, but it could have a lot of other factors contributing to it. Your soil test will have a lot of valuable information on this. Don't overlook the role that species and cultivar play here. A common bermudagrass lawn will never look quite the same as an improved common or a hybrid lawn, no matter what kind or how much fert you apply.